Quantitation of the number of subunits in an oligomeric protein can be challenging, particularly if multiple forms are present. It is not uncommon for the oligomeric state to be determined for one member of a protein family and then assigned to other members without direct verification. This situation reflects the experimental difficulties associated with unambiguous quantitation of the number of subunits in an oligomer, particularly if multiple species are present. In addition, membrane proteins pose special problems for determination of oligomeric states due to complications arising from the presence of lipid and detergent molecules, and techniques such as cross-linking as reported by Azem et al., 1995, Biochimica et Biophysica Acta, 1243:151-156, single molecule subunit counting, as reported by Ulbrich and Isacoff, 2007, Nat Methods, 4:319-321, or mass spectroscopy as reported by Barrera et al., 2009, Nat Methods 6:585-587, involve difficulties in interpretation or require specialized equipment. Accordingly, there is no gold standard for membrane protein oligomer characterization.